Island gypsy 36 tank

[Fishindave]

Hi All I just bought an Island gypsy 36 quad and noticed a tank located aft of the generator. It’s pretty big and hidden. The previous owner never told me about it and I’m wondering if it’s water or fuel. There are no deck fills, valves etc to it that I can find anywhere. Any feedback would be appreciated

 

[Comodave]

Welcome aboard. I guess that I would find any hoses going to the tan and follow them and see where they go to. It could be either and fill off a second tank, but that isn’t a very efficient way to fill a tank. What material is the tank made out of?